Chapter 28 – Stars and Galaxies

Page 610 Why is this galaxy so bright? What shape is this galaxy? How do we obtain images like this?

Chapter 28.1

A Closer Look at Light

What is light?

Light is a form of electromagnetic radiation (EM)

Other types of EM listed from longest to shortest wavelengths Radio Microwave Infrared Visible light Ultraviolet X-rays Gamma rays

This is known as the EM spectrum

All EM energy travels in waves and at the speed of light

Can travel through empty space (vacuum)

Spectroscope

Visible white light is actually made up of light of various colors each with a different wavelength

The various color can be observed In a rainbow Passing light through a prism Or an astronomers instrument known as a

spectroscope

The colors ROY G. BIV are aligned from longest to shortest (violet)

Longer wavelengths refract less

Chapter 28.2

Types of spectra from spectroscopes

Continuous spectrum – unbroken band of colors which are emitted by Glowing solids (filament) Glowing liquids (molten iron) Hot compressed gases (inside stars)

Emission spectrum – lines of different colors Produced by glowing thin gases

Each element has its own spectra therefore scientists can identify the gas

Absorption spectrum – a continuous spectrum crossed by dark lines Elements in the thin gas that surround a

star absorb the same wavelength they would emit

The stars absorption spectrum indicates the composition of the stars outer layer

The sun radiates a continuous spectrum, however, the gases in the atmosphere absorb some wavelengths. By analyzing the absorbed bands, scientists can figure out the composition of the sun’s outer layer.

Absorption spectrum can also determine a planets atmosphere

Doppler Effect

By observing how the spectral lines are shifting, scientists can tell how a star is moving compared to the earth Move to the red end, “red shift,” the star is

moving away Move to the blue end, “blue shift,” star is

getting closer By using spectrums from the lab and the

stars, they can determine how fast they are moving

Chapter 28.3

Stars and Their Characteristics

Constellations – name given to groups of stars

There are 88 of them Big dipper – best known asterism

(small star grouping) is actually part of a larger grouping known as Ursa Major

We can use the dipper to find other constellations

The stars appear to move in two ways Nightly variation – due to earth’s

rotation Yearly due to earth’s revolution

Orion – winter constellation Lyra – summer constellation

Apparent magnitude

Is the brightness of the star as seen from earth

The lower the number, the brighter it is Sun is –26.7 Faintest is +6

Distance to stars Astronomical Unit (AU) distance from earth to sun –

150 million km Proxima Centauri – next nearest star is 260,000AU Therefore, astronomers use light-years – a unit of

measure that light travels in one year. Light travels about 300,000km/sec 1 year =

9.5×1012

Proxima Centauri is about 4.2 light years away

Elements in Stars

Stars are mostly of super-hot gases – mostly H & He

Mass, Size and Temperature of Stars

Mass is something that can not be observed directly. It can only be calculated based on other observations

Stellar mass is expressed as multiples of the sun’s mass Betelgeuse’s mass – 20 solar masses

Temperature and Color

Blue stars are hot Red stars are cool

Luminosity The actual brightness of the star is luminosity If two stars have the same surface temperature, the

larger star would be more luminous If the same size, hotter one would be brighter Types of magnitude

Absolute – as if all stars were same distance from earth

Apparent – as they appear in the nighttime sky

Variable Stars

Some stars show regular variation of brightness over cycles that last from days to years

Cepheid Variables – yellow supergiants whose cycles range from 1 to 50 days If a Cepheid is located in another galaxy,

astronomers can find the distance to these galaxies by comparing absolute and apparent magnitudes

Other stars change in brightness because they revolve around another star. This is known as a ‘binary star system.’

Chapter 28.4

Life Cycles of Stars

Hertzsprung-Russel Diagram

A diagram to help explain a stars life Most stars fall into 4 distinct groups

Main sequence – 90% of stars Our sun

Giants – 10-100x bigger than our sun – more luminous

Supergiants – more than 100x bigger than our sun

White dwarves – stars near the end of their lives

Birth of a Star

Stars begin their life as a nebula Huge cloud of gas

Death of a Star

2 paths Nebula protostar main sequence

red giant planetary nebula white dwarf Nebula protostar massive star red

supergiant supernova blackhole or neutron star

Our sun will swell into a red giant, then its outer layers will get blown away and only an earth-sized fiery hot carbon-oxygen core will remain (white dwarf)

Remnants of Massive Stars

Massive star goes supernova, the core that is left behind is so massive that the electrons of elements are pulled into the nucleus forming a neutron star.

Some neutron stars spin rapidly, burst of radio waves, these are known as pulsars

Black hole – so dense that their gravity will not even let light escape

Chapter 28.5

Galaxies and the Universe

What are galaxies

A system of millions of stars that appear as a single star in our sky

There is between 50-100 billion galaxies There are millions of light years between

galaxies Sun belongs to the Milky Way Galaxy

which is a spiral galaxy Milky Way belongs to the Local Group of

about 30 galaxies

Types of galaxies

Spiral – like the Milky Way Elliptical – spherical to lens shaped Irregular – much smaller and fainter

with no shape

Active Galaxies

Galaxies that emit more energy than their combined stars are said to be active

Currently it is thought that super massive black holes are at the center of these galaxies